The Defiance of Duskendale
Lord Denys Darklyn seized King Aerys II inside his own town walls and held him for half a year. Barristan Selmy went over the wall alone and brought the king out; House Darklyn was extinguished root and branch. Aerys went in eccentric and came out something else ÔÇö the Mad King's reign is usually dated from this half-year in a cell.
The fuller tale
Lord Denys Darklyn of Duskendale wanted a charter for his town ÔÇö the right to hold its own market and keep its own customs ÔÇö and the crown refused. So when King Aerys II rode down in person to settle the argument, Darklyn shut the gates behind him and kept him.
The siege lasted half a year. Tywin Lannister, Hand of the King, brought an army and then did nothing with it, on the stated ground that storming the town would get the king killed and on the unstated ground that a dead king would leave him governing for the boy. Aerys sat in a cell under Duskendale for six months listening to that argument being had outside the walls.
It was ended by one man. Ser Barristan Selmy of the Kingsguard went over the wall alone at night, cut his way to the dungeon, and brought the king out. Duskendale was then sacked, House Darklyn was extinguished root and branch, and Lady Serala, whom the singers blamed for the whole business because she was foreign, was dealt with in a manner the histories describe and this chronicle will not repeat.
The man who came out
Aerys went into that cell eccentric and came out something else. He had been an energetic, ambitious young king; afterward he would not let a razor near him, would not leave the Red Keep for years, saw treason in his Hand, his son, his wife and his own hands, and began to take an interest in fire that was not a hobby.
The maesters date the reign of the Mad King from these six months. Whether the cell caused it or revealed it cannot be answered, but everything that follows in this chapter ÔÇö the wildfire caches, the burnings, the two men dead on the throne room floor ÔÇö runs from a lord who wanted a market charter and a Hand who saw an opportunity in waiting.
Elsewhere in the wiki
 See the world as it stood in 277 AC What became of it  if an ending is written, it is here. Spoilers.
The chronicle records no ending here ÔÇö or none the maesters have set down.